On-ground management actions targeting invasive and/or native species are often undertaken based on incomplete and biased distribution data for the species of interest. Once an invasive species becomes established, containment can provide an effective management option to conserve native biodiversity only if it is implemented beyond the outer distribution limits of the species of interest. Determining these outer distribution limits is currently difficult for freshwater fish species because of the low sensitivity and biases associated with conventional monitoring methods. The improved sensitivity of environmental DNA-based surveys makes them particularly useful to determine these outer distribution limits. In this study, we used conventional monitoring methods and eDNA-based monitoring using real-time PCR to determine the spread of the invasive redfin perch (Perca fluviatilis) in an intermittent river system. This voracious predatory fish is responsible for the continued decline of several threatened and vulnerable species within Australia. We found that eDNA detection rates were high in our study system, when redfin perch presence was confirmed by conventional monitoring, compared to previously published works. Additionally we describe how the combination of conventional and eDNA-based monitoring can improve redfin perch distribution data compared to conventional monitoring alone. This improvement has subsequently been used to inform management and determine the optimal location for the construction of an exclusion barrier.